Method for anodizing aluminized steel strip

ABSTRACT

Aluminized steel strip is anodized during movement through a succession of anodizing tanks, each housing at least one anodizing stage. The strip is subjected to successively higher current densities in successive anodizing stages. The current density in the first anodizing stage is controlled to prevent pitting, and the increase in current density from one anodizing stage to the next is relatively large.

I United States Patent 1151 3,650,910 Froman 1 Mar. 21, 1972 [54] METHOD FOR ANODIZING 2,977,294 3/1961 Franklin ..204/58 x ALUMINIZED STEEL STRIP 3,020,219 2/1960 Franklin et al. .....204/58 3,074,857 1/1963 Tenpohl ..204/28 Inventory Fromm Lansing, 3,079,30s 2/1963 Ranivez et al. ..204/58 Assignee: Inland p y Chi ag Rosenthal et al. [22] Filed: Nov. 19, 1970 Primary Examiner-1 C. Edmundson [2]] App! No 90 892 Anarney--Merriam, Marshall, Shapiro & Klose [57] ABSTRACT 5 2 53: Aluminized steel strip is anodized during movement through a succession of anodizing tanks, each housing at least one [58] Field of Search ..204/28, 56, 58 anodizing a The strip is subjected to successively higher current densities in successive anodizing stages. The current [56] References cued density in the first anodizing stage is controlled to prevent UNITED STATES PATENTS pitting, and the increase in current density from one anodizing stage to the next is relatively large. 2,839,455 6/1958 La Tour et a] ..204/28 2,905,600 9/ l 959 Franklin ..204/58 X 9 Claims, 2 Drawing Figures /0 (/2 /0 l0 K (9) l2 /2 2 l2 l2 l2 I2 1 i '5 E- i- 1: 11 1 m i f 1 22 30. DRYING O O Q o o 1 /5 l3 v3 /3 l9 /3 2/ 23 24 CLEANING ANODIZING 20 SEALING PATENTEDMARZ] m2 CLEANING gig-2 BY I INVENTOR ATTORNEYS BACKGROUND OF THE INVENTION The present invention relates generally to the anodizing of aluminized steel strip and more particularly to a continuous process for anodizing aluminized steel strip without subjecting the strip to adverse attack thereon during the anodizing process. 7

Aluminized steel is a composite metal product having a steel base and an aluminum coating. Anodizing is a process in which aluminum is electrolytically coated with a layer of aluminum oxide to impart certain improved properties to the aluminum, such as increased corrosion resistance, increased abrasion resistance, etc. Aluminized steel strip stains when subjected to severe corrosive conditions. Anodizing the aluminized steel strip prevents such staining.

A continuous process for anodizing aluminum strip (as distinguished from aluminized steel strip) is disclosed in Ramirez et al. US. Pat. No. 3,079,308 and in an article entitled High Speed Continuous Anodizing of Aluminum" by Erik F. Barkman appearing in PROCEEDINGS, AMERICAN ELECTROPLATERS SOCIETY, Vol. 51, pp. 125-132 (1964). A process for anodizing aluminized steel is disclosed in LaTour et al. US. Pat. No. 2,839,455.

In a conventional anodizing process, the article having an aluminum surface undergoing anodizing is immersed in a liquid electrolyte (e.g., 10 volume percent sulfuric acid). Also immersed in the solution are metallic cathodes. The article undergoing anodizing is the anode of an electrolytic cell; and, when the cell is connected to a source of DC current, a layer of aluminum oxide is formed on the aluminum surface of the article.

A problem which occurs during the anodizing of aluminized steel strip is pitting or erosion in localized surface areas of the aluminized steel. The aluminum coating may unavoidably contain up to 3 percent iron, carried over from the coating bath of molten aluminum which dissolves iron from the steel strip, up to the equilibrium solubility of iron in the bath; and the aluminum coating may contain up to 10 percent silicon as an alloying element. Other iron-containing constituents, such as Fe Al or FeAl (commonly referred to as dross"), may be suspended in the aluminum coating.

Pitting occurs during anodizing because of the presence, in the aluminum coating, of small, discrete particles composed of or containing iron or silicon. During the anodizing process, these particles may be selectively dissolved or oxidized at a rate different than the aluminum alloy. The electric current in the anodizing cell is initially directed relatively uniformly towards the entire aluminized surface, whereupon an electrically insulating aluminum oxide layer is formed. If those particles, at or near the aluminum surface, do not oxidize at the same rate as the surrounding aluminum alloy, the electric current will then concentrate toward these particles because they are the points of least electrical resistance. Increases in current density at such areas, on the order of 10', could possibly result; and the intense heat from such a concentration of current causes local thermal breakdown of the coating, and the formation of pores through the aluminum to the base steel may result.

Another problem occurs when gas bubbles (e.g., hydrogen or oxygen), unavoidably generated during the anodizing operation, accumulate on the surface of the article being anodized. This can occur, for example, when a strip undergoing anodizing is horizontally disposed in the anodizing tank, thus blocking the upward movement of the naturally rising gas bubbles which accumulate along the bottom surface of the horizontal strip. Bubbles on the aluminum surface electrically insulate the underlying metal, and the current is concentrated at gaps in this insulation, causing localized over-heating of the aluminum coating and the resulting destruction thereof. The gas bubble accumulation on the anodized article can be reduced by mechanical or forced air agitation.

SUMMARY OF THEINVENTION A method in accordance with the present invention provides a continuous process for anodizing aluminized steel strip in which the anodized coating is formed in two parts, with a tenacious barrier layer of oxide being formed under a first set of anodizing conditions and with additional oxide being formed, after the formation of the barrier layer, under different anodizing conditions. The oxide coating is formed while the aluminized steel strip moves continuously through a succession of anodizing stages.

The barrier layer is formed in 2.5-5 seconds in a first anodizing stage. Concentration of current towards iron or iron-containing particles at or near the surface of the aluminum coating is avoided during formation of the barrier layer by controlling the current density (current per given surface area of coating), after the unavoidable initial surge of current, to no greater than 15 amps/ft". After the barrier layer forms, the iron particles are sufiiciently insulated to avoid current concentration threat.

After formation of the barrier layer, the current density is increased, in subsequent anodizing stages, relatively rapidly to a terminal current density in the range -1000 amps/ft. This occurs in about 5-10 seconds.

Problems arising from the generation of gas bubbles are minimized by moving the aluminized strip substantially vertically through the anodizing stages and at a speed which causes mechanical agitation of the electrolyte. Additional agitation (e.g., with air) would be necessary if the vertical speed does 'not cause sufficient agitation or if the anodizing process is operated with the strip in a horizontal disposition.

The percentage increase in current density during the formation of additional oxide exceeds 35 percent from anodizing stage to anodizing stage. This large incremental increase in current density helps reduce the extent to which the aluminized steel sheet is subjected to attack during the formation of additional oxide.

Other features and advantages are inherent in the method claimed and disclosed or will become apparent to those skilled in the art from the following detailed description in conjunction with the accompanying diagrammatic drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1. is a diagrammatic view of apparatus for performing an anodizing method in accordance with an embodiment of the present invention; and

FIG. 2. is a schematic of part of the electrical system used in the anodizing apparatus of FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 1, a continuous strip 10 of aluminized steel strip from a coil 11 moves substantially vertically through a series of tanks 15-21. Strip 10 moves substantially vertically through each of said tanks by being passed over upper rollers 12, located above each tank 15-21, and under lower rollers 13 located within and near the bottom of each tank 15-21.

Aluminized steel strip 10 is cleaned in tanks 15, 16 and 17, then anodized in tanks 18, 19 and 20, following which the anodized aluminized steel strip is subjected to a sealing operation in tank 21. After sealing, strip 10 passes under a roller 22 and into a drying stage 23 following which the strip is coiled at 24.

The composition of strip 10 is that of conventional aluminized steel strip, both for the steel base and the aluminum coating. Conventionally, the aluminum coating may be either of two types: one type which contains up to 10 percent silicon for use where improved heat resistance is desired; and another type, without silicon, where increased corrosion resistance is desired.

Cleaning tank 15 houses an electrolytic cleaning stage employing two pairs of anodes 30 arranged in vertical, parallel disposition with each anode in a pair located on an opposite side of strip 10 as it passes initially downwardly and then upwardly through tank 15. Strip 10 has a negative charge relative to the anodes in tank 15, but the charge is positive relative to the cathodes in anodizing tanks 18, 19 and 20, as will be explained subsequently. The liquid electrolyte in tank 15 is a conventional inhibited alkaline solution used for electrolytic cleaning of aluminum. The purpose of the electrolytic cleaning step is to remove dirt, oil, grease, oxide and the like.

Tank 16 contains water for rinsing strip 10 to remove electrolytic cleaning solution remaining on the strip from its passage through tank 15.

Tank 17 contains a conventional etch solution to remove undesirable films, etc. from the surface of the aluminum coating on strip 10 and to etch the surface of the aluminum to render it more readily anodizable. Typically the solution in tank 17 is wt. percent sodium hydroxide.

After the strip leaves etch tank 17 it should be rinsed in a tank of water (not shown but like tank 16). Other conventional cleaning stages, not shown, may be interposed between the last cleaning tank 17 and the first anodizing tank 18.

After being cleaned, strip is passed through a first anodizing tank 18 in which a tenacious barrier layer of aluminum oxide is formed on the aluminum coating of aluminized steel strip 10. Tank 18 includes two pairs of vertical, parallel cathodes 31, each member ofa pair being arranged on an opposite side of strip 10 as it moves first vertically downwardly and then vertically upwardly through tank 18.

The electrolyte in tank 18 is 10 volume percent sulfuric acid 18 wt. percent), conventionally utilized for the anodizing of aluminum.

The current density maintained in tank 18, after the unavoidable initial surge of current, is no greater than amps/ft? for the entire time the strip is in tank 18. This is a time in the range 2.5-5 seconds, with about 3 seconds being preferable. These time and current density conditions are sufficient to form the desired tenacious barrier layer of aluminum oxide; and controlling the current density in the manner described prevents pitting of the aluminized steel strip.

After strip 10 leaves tank 18, additional oxide is formed over the barrier layer, in subsequent anodizing stages in tanks 19 and 20.

Strip 10 moves substantially vertically through anodizing tanks 19 and 20, and the cathodes 32 (tank 19) and 33, 34 (tank 20) are in a substantially vertical disposition.

Anodizing tank 19 contains two pairs of parallel, vertical cathodes 32 all of which are connected together to maintain a single current density within tank 19. Typically this current density is about 50 amps/ft. and the time during which strip 10 is subjected to this current density in tank 19 is about 3 seconds.

After leaving anodizing tank 19, strip 10 enters anodizing tank 20 which, in the illustrated embodiment, houses two anodizing stages. One anodizing stage utilizes a first pair of parallel, vertical cathodes 33, 33; and the second anodizing stage utilizes a second pair of parallel, vertical cathodes 34, 34.

Typically, the current density at the anodizing stage defined by cathodes 33, 33 is about 100 amps/ft. and the time in which strip 10 is subjected to this current density is about 2 seconds. Typically, the current density in the anodizing stage defined by cathodes 34, 34 is in the range l35l,000 amps./ft. preferably in excess of 250 amps/ft; and the time in which strip 10 is subjected to a current density in the range l35-l ,000 amps/ft. is about 2 seconds.

In the illustrated embodiment, no anodizing occurs after strip 10 leaves tank 20, and the current density applied at cathodes 34 is the terminal current density. If desired, however, the anodizing operation can be conducted in more than four stages (one stage for formation of the barrier layer and three stages for formation of additional oxide) illustrated in FIG. 1.

After anodizing, the anodized steel strip 10 may be subjected to a sealing step for optimizing the stain resistant properties of the oxide coating. Sealing tank 21 contains hot water (e.g., 200-2l2 F.) slightly acidic with a pH in the range 6-6.5; and, as strip 10 passes through tank 21, the oxide coating thereon is sealed.

Sealing may be eliminated by controlling the concentration of the electrolyte in the anodizing tanks and by controlling the terminal current density. For example, if the electrolyte is 10 volume percent sulfuric acid and the terminal current density exceeds 250 amps./ft. the sealing step may be eliminated. A higher concentration of sulfuric acid or a terminal current density below 250 amps/ft, or both, would require a sealing operation for strip 10.

After sealing tank 21 (or after the last anodizing stage if scaling is eliminated) strip 10 passes under a roller 22 and through a drying stage 23 in which a hot air blast is directed against strip 10 to dry the strip, following which the strip is coiled at 24.

During anodizing, gas bubbles (typically oxygen and hydrogen) are generated adjacent strip 10. If these bubbles move in the same direction and at the same speed as does the strip, they may form an insulation layer at various locations on the surface of the strip, with gaps therein. The current tends to concentrate at the gaps in these insulation layers, causing an undesirable attack on the aluminized coating.

In a method in accordance with the present invention, gas bubbles are prevented from accumulating as an insulation layer on the aluminized steel strip. More specifically, strip 10 is moving downwardly and upwardly in a given anodizing tank at a speed (e.g., 30 feet per minute) which causes sufficient agitation of the electrolyte to prevent gas bubbles from accumulating on the strip. The faster the strip moves, the more agitation there is. Moreover, the faster the strip speed, the more desirable it is from an economic and production standpoint. If strip 10 moves horizontally through the anodizing stages, additional agitation may be required.

As described above, the current density is increased, during the formation of additional oxide in tanks 19, 20 from no greater than 15 amps/ft? (the current density maintained throughout the formation of the barrier layer of oxide) to a terminal current density in the range -l,000 amps./ft. The increase in current density occurs in 7 seconds, 3 seconds in tank 19 and 4 seconds in tank 20 (2 seconds for each of the two stages in tank 20). In preferred embodiments, a ermissible range of time during which this increase in current density occurs is about 5-10 seconds. Longer anodizing times are possible, but not so desirable from a commercially practicable standpoint. The shorter the time required for anodizing, the more attractive the process from a commercial standpoint.

The length of time during which the strip is subjected to a given current density in a given tank can be varied by varying the speed of the strip, by varying the electrolyte level in an anodizing tank or by varying the vertical dimension of the cathodes in a tank. Typically, strip 10 is moving at a speed of about 30 ft./minute.

The formation of additional oxide and the increase in current density is accomplished in three different anodizing stages, one stage being in tank 19 and two stages in tank 20. The formation of additional oxide should take place in at least two anodizing stages following the stage in which the barrier layer is formed; and the increase in current density from one anodizing stage to the next should be at least 35 percent. If the increase in current density from one anodizing stage to the next is too gradual, e.g., 30 percent or less, the time required to form the necessary thickness of oxide on the strip is increased and the commercial attractiveness of the process decreases. Moreover, a rapid increase in current density from one anodizing stage to the next appears to contribute to a lessening of attack on the aluminized steel strip 10 during the anodizing operation.

Cathodes 31, 32, 33, 34 are of conventional construction and are typically composed of lead or carbon. Anodes 30 in electrolytic cleaning tank 15 are of conventional steel construction.

The electrolyte in all the anodizing tanks 18, 19, 20 is sulfuric acid having a concentration of about 8.5- vol. percent.

Referring now to FIG. 2, electrolytic cleaning anodes 30 are connected to the positive sides of rectifiers 35, 36, 37 and 38. The negative side of rectifier 35 is connected to cathodes 31 in anodizing tank 18 for forming the barrier layer of oxide coating. The negative side of rectifier 36 is connected to cathodes 32 in anodizing tank 19 for forming additional oxide coating on the steel strip. The negative side of rectifier 37 is connected to cathodes 33 in tank 20; and the negative side of rectifier 38 is connected to cathodes 34 in tank 20. With the arrangement of F l6. 2, any of the rectifiers 35-38 can be adjusted to vary the current density at the particular anodizing stage to which it is connected without changing the current density at any other anodizing stage. In the illustrated embodiment, voltage is in the range 12-30 volts.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art. a

What is claimed is:

l. A method for anodizing aluminized steel strip, said method comprising the steps of:

moving a continuous strip of steel, having a surface coated with aluminum, through a series of anodizing stages each having at least a pair of cathodes;

forming a barrier layer of oxide on the surface of the aluminum coating, in a first anodizing stage having said cathodes, for a time of 2.5-5 seconds;

controlling the current density in said first anodizing stage,

after the initial surge of current, to no greater than amps/ft. for the entire time said strip is in said first anodizing stage, to deter pitting;

forming additional oxide over said barrier layer in a least one subsequent anodizing stage;

and increasing the current density, during the formation of said additional oxide, to a terminal current density of amps/.ft. to about 1 ,000 amps/ft.

2. A method as recited in claim 1 wherein:

said strip moves substantially vertically and the cathodes are in a substantially vertical disposition, in all of said anodizing stages.

3. A method as recited in claim 2 wherein:

the speed of said strip is high enough to cause agitation of the electrolyte, in the anodizing stages, sufficient to prevent gas bubbles, generated during the anodizing operation, from accumulating on said strip.

4. A method as recited claim 1 wherein:

said current density is increased, during the formation of said additional oxide, from about 15 amps/ft. to a terminal current density of 135-1 ,000 amps/ft.

5. A method as recited in claim 4 wherein:

said current density is increased to said terminal current density in three stages.

6. A method as recited in claim 4 wherein:

said increase in current density occurs in a time of about 5-10 seconds.

7. A method as recited in claim 6 wherein:

said increase in current density occurs in a time of about 7 seconds.

8. A method as recited in claim 1 wherein:

said additional oxide is formed in at least two subsequent anodizing stages;

and the increase in current density from one anodizing stage to the next is in increments of at least 35 percent.

9. A method as recited in claim 1 and operating with an electrolyte of sulfuric acid having a concentration 8.5-10 vol. percent;

and a terminal current density of 250 amps/ft. to about 1,000 amps/ft. eliminates the need for a sealing step. 

2. A method as recited in claim 1 wherein: said strip moves substantially vertically and the cathodes are in a substantially vertical disposition, in alL of said anodizing stages.
 3. A method as recited in claim 2 wherein: the speed of said strip is high enough to cause agitation of the electrolyte, in the anodizing stages, sufficient to prevent gas bubbles, generated during the anodizing operation, from accumulating on said strip.
 4. A method as recited claim 1 wherein: said current density is increased, during the formation of said additional oxide, from about 15 amps./ft.2 to a terminal current density of 135-1,000 amps./ft.2.
 5. A method as recited in claim 4 wherein: said current density is increased to said terminal current density in three stages.
 6. A method as recited in claim 4 wherein: said increase in current density occurs in a time of about 5-10 seconds.
 7. A method as recited in claim 6 wherein: said increase in current density occurs in a time of about 7 seconds.
 8. A method as recited in claim 1 wherein: said additional oxide is formed in at least two subsequent anodizing stages; and the increase in current density from one anodizing stage to the next is in increments of at least 35 percent.
 9. A method as recited in claim 1 and operating with an electrolyte of sulfuric acid having a concentration 8.5-10 vol. percent; and a terminal current density of 250 amps./ft.2 to about 1,000 amps./ft.2 eliminates the need for a sealing step. 